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This invention relates to a control system for controlling the working depth of a tractor-coupled implement as a function of various sensed and operator-controlled parameters.
Various electrohydraulic hitch control systems have been designed and built or proposed. Various attempts have been made to improve hitch control system performance utilizing electronics and/or microprocessors. See, for example, U.S. Pat. Nos. 4,508,176; 4,518,004; 4,503,916 and 4,837,691. However, such systems utilizing electronics have not generally been fully utilized due to factors such as complexity of the operator controls. A known production hitch control system, such as provided with John Deere tractors, includes a rockshaft control lever which is movable to raise and lower the hitch under certain conditions and to set implement working depth. This known system also includes a cab mounted raise/lower switch. When the cab switch is in the "down" position, the control system operates to control implement depth around the working depth set by the control lever and the control system responds to changes in the setting of the control lever. When the cab switch is in the "up" position the setting of the control lever has no influence and the hitch and implement are moved to their fully up positions. In this system, if there is a failure of the rockshaft lever potentiometer, then the control system will lock the hitch and implement at their current position. If there is a failure of the hitch position feedback potentiometer, then the "up" position of the cab switch may cause the hitch to move to its fully up position and the "down" switch position may cause the hitch to move to its fully lowered position. This known production system also includes an external switch mounted on the rear of the tractor near the hitch. This external switch can be manipulated to raise or lower the hitch to any position desired by the operator. The cab switch is a two-position detent held type switch and the external switch is a three position switch, spring biased to its center position.
Another known production hitch control system, such as is available with the Massey-Ferguson tractors, includes a rockshaft control knob which sets the implement working depth and a three-position detent held lift/lowering switch with up, down and neutral positions. This system operates similar to the previously described system when the three-position switch is in its "up" and "down" positions. However, when the three-position switch is in its "neutral" or center position, then the control system is disabled, hitch movement is prevented and the system does not respond to movement of the rockshaft control knob. In both of these known systems, the raise/lower switches are detent-held type switches which remain in whatever position they are placed in until intentionally moved out of such position. With either of these known hitch control systems, to raise or lower the hitch to a desired position with the control lever/knob requires a correct combination of control settings. The raise/lower switch must be in its down position or the control lever will not function. A failure of this switch in the up position can disable the function of the control lever. It would be desirable to provide a hitch control system which requires fewer operator actions, and which has other performance advantages made possible by electronics.